The present invention relates to a printer for printing on recording paper comprising a platen, a head, a spring member, and an opening/closing operation member. More particularly, the present invention relates to a mechanism for pressing a head against a platen and an opening/closing mechanism.
A general construction of a conventional printer is now described briefly in the following. As shown in FIG. 4A, a printer comprises a platen 101 and a thermal head 102. The platen 101 is rotatably supported about an axis 101a along the width direction of recording paper (not shown). More specifically, a stepping motor 104 is connected with the axis 101a via a train of gears 103. The rotational movement of the stepping motor 104 is decelerated by the train of gears 103 and is transmitted to a rear axis 101a, and the platen 101 is appropriately intermittently rotated for paper feed in the direction shown by an arrow in the figure. The thermal head 102 is disposed so as to face the platen 101 from behind via the recording paper. The thermal head 102 is swingably supported on an axis 105. During a printing operation, a printing portion of the thermal head 102 is pressed against the recording paper. With this state maintained, the printing portion is electrically energized to print a line of letters on the recording paper. After the printing of the line, the platen 101 is rotated in the direction shown by the arrow to feed the recording paper. FIG. 4B shows a schematic cross-sectional structure of the printer shown in FIG. 4A. As shown in the figure, the thermal head 102 is disposed so as to face the platen 101 from behind via recording paper 106. When the thermal head 102 is swung in a forward direction (counterclockwise in the figure) about the axis 105 which is in parallel with but different from the axis 101a on the side of the platen 101, the printing portion above the axis 105 is pressed against the platen 101. In order to provide the pressing force, a spring member 107 intervenes between the thermal head 102 and a frame 108 of the printer. On the contrary, when the thermal head 102 is swung in a backward direction (clockwise in the figure) against the urging force by the spring member 107, the printing portion of the thermal head 102 is retracted from the platen 101. This operation is carried out when, for example, the recording paper 106 is fed between the platen 101 and the thermal head 102.
In the conventional printer, the spring member 107 is engaged with a portion of the thermal head 102 above the axis 105, and the pressing force of the spring member 107 due to its resiliency presses the printing portion of the thermal head 102 against the platen 101. The spring member 107 presses, for example, a portion in the middle of the paper width of the thermal head 102. However, with this construction, it is difficult to press evenly the printing portion of the thermal head 102 against the platen 101 along the width direction of the platen 101, and a working face between the printing portion and the platen is often one-sided. This causes uneven density of printing and partially blurred printing on the recording paper 106. In particular, when, for example, the platen 101 is not in parallel with the thermal head 102, it is difficult to press evenly the printing portion of the thermal head 102 against the platen 101 with the spring member 107. Further, during a printing condition, the thermal head 102 is constantly pressed against the platen 101 with considerable pressing force. In order to bear this pressing force, the frame 108 is required to have mechanical strength to some extent, and thus, it is difficult to design a smaller and lighter printer. Particularly, when, for example, the recording paper 106 is fed, the thermal head 102 has to be retracted from the platen 101 against the pressing force by the spring member 107. Since great force acts in this operation, the frame 108 is required to have enough mechanical strength to bear the force.